The invention relates to a process for applying a coating of ceramic material to a substrate by plasma spraying wherein the material is added to the plasma jet and includes a chemical compound, one constituent of which is a non-metallic element from the group N, C, B or from the main groups VI or VII of the Periodic Table which decomposes at least partially before reaching the melting point and is present in the solid phase in the applied state.
Such a process is known from DE-OS 30 24 611. Herein, iron spinel and cobalt spinel are applied by plasma spraying at low torch power. In view of the low torch power, it is to be assumed that only the iron spinel will melt as it has a low melting point, while the cobalt spinel will only be embedded in the molten iron spinel. Furthermore, decomposition products of the cobalt spinel will presumably be present in spite of the low torch power.
With plasma spraying at low torch power, the bonding of the applied coating to the substrate is not optimum and the internal solidity of the outer coating is also limited. Furthermore, the coating yield is also very low for when the material to be sprayed is hurled in the still solid state by the plasma jet against the substrate, an impact reflection occurs at the substrate and hence only a small part of the material to be sprayed remains adhering to the substrate.
The object underlying the invention is, therefore, to so improve a process of the generic kind that the chemical compound contained in the material can be applied stoichiometrically, i.e., undecomposed to the substrate and forms an impermeable, adherent and stable coating.